Surface improving process

ABSTRACT

The uneven surfaces of wood-based materials such as chipboard may be improved by applying a formable coating material, smoothing and/or modelling the coating material to achieve a smoother surface, and heating the coating material to form closed pores while maintaining the smoothed surface intact. Decorative and flexible layers may also be applied to such surfaces in this manner.

[0001] This invention relates to a process and an assembly for improvingsurfaces, preferably uneven narrow surfaces, of wood-based materials.

[0002] Surface treatment processes of the type in question are widelyused in the furniture industry. This is because the wood-based materialsused in the furniture industry are not left in their original state, butare given an improving surface coating, mostly a melamine coating. Thecoating materials used also include veneers, decorative laminates ordecorative films.

[0003] A special edge material is often used for coating sides, edgesor, generally, narrow surfaces. This special edge material may be, forexample, an edge veneer or a so-called narrow-surface band which isapplied to the narrow surfaces by means of an adhesive. This process isknown generally as edge banding.

[0004] However, it is often derisable to round off the narrow surfacesor edges on aesthetic grounds or to provide profiled surfaces forfunctional reasons. Other coating techniques besides edge banding, whichis also known as soft forming, are used in the coating of such “soft”contours.

[0005] One of the reasons for wanting to improve surfaces is that thesurfaces to be coated are often very uneven, full of voids, etc. Thisapplies in particular to chipboards.

[0006] When coated by one of the processes mentioned above, surfacessuch as these become noticeably uneven, “bumpy” or undulating whichspoils the appearance of subsequent pieces of furniture for example. Toobtain an improvement, thicker edge or coating materials are often usedbecause they are able sufficiently to level out any unevenness in thesurface and to avoid so-called telegraphing of the substrate. However,any increase in the thickness of the material is accompanied by anincrease in the resilience of the bent coating material. This in turnmeans that a longer contact pressure zone is required for bonding thestiffer coating material. Moreover, thicker coating materials aregenerally more expensive.

[0007] The process and assembly described in DE 44 42 397 for treatingsurfaces, more especially narrow surfaces, are better. Here, a formablecoating material is applied and smoothed by means of a smooting band.However, the surfaces obtained are again not smooth enough for allapplications.

[0008] Accordingly, the problem addressed by the invention was toprovide a possibility by which the disadvantages mentioned above couldbe eliminated and which would enable surfaces to be smoothed in a simplemanner.

[0009] This problem has been solved by a process comprising the stepsmentioned in claim 1 and by an assembly with the features defined inclaim 7.

[0010] The coating material is applied, for example, by a roller, anadjustable slot die or a transfer tape, preferably to the edge band(edge veneer). However, it may also be applied to the wood-basedmaterial. It is preferably applied to the surface by means of a transfertape. This has the advantage that there is no need for an applicatorroller adapted to the profile to be coated in order to apply thematerial. The manufacture of such profile-adapted applicator rollers oreven specially adapted application nozzles is time-consuming andextremely expensive, especially where large numbers of different edgeprofiles are involved. In addition, different profiles cannot be coatedin a short time.

[0011] The coating material is preferably applied to the surface to betreated by pressure. A suitable pressure element applies a force to theband on its uncoated side so that the coated side of the transfer tapeis pressed onto the surface to be coated.

[0012] Application of the coating material is improved by additionalheat treatment of the coating material or wood-based material. Infraredheaters, hot air fans, high-frequency heaters or the like are preferablyused for this purpose.

[0013] In one preferred embodiment of the process/assembly according tothe invention, the coating material is applied to the transfer tapeimmediately before application to the surface to be coated. This ispreferably done using a suitable applicator roller or an applicationnozzle which applies the material to the transfer tape in an adjustablethickness. In determining the material thickness, it is of advantage toincrease it in the middle part of the transfer tape. As mentioned above,surface unevenness is at its greatest in particular in this region ofthe surface to be coated with the result that more material has to beused there.

[0014] In another advantageous embodiment of the invention, the transfertape is removed from the surface to be treated, the material havingpreviously penetrated into and set or hardened in the pores of thesurface to be treated so that it does not stick to the transfer tapewhen it is removed.

[0015] Another advantageous embodiment of the invention is characterizedin that, after removal, the transfer tape is returned to the applicationsystem so that an endless tape can be used.

[0016] The transfer tape is preferably also used as a smoothing tape sothat there is no need for smoothing rollers or smoothing blocks.

[0017] However, smoothing or modelling can also be carried out by otherknown methods, for example by rollers and coating knives or by sandingafter spray application.

[0018] Further information on application and smoothing can be found inthe relevant literature, cf. in particular DE 44 42 397 to whichreference is expressly made.

[0019] The coating materials used may be both solid and non-solid, forexample thixotropic, paste-like or highly viscous, at room temperature.

[0020] Where importance is attached to simple processing and not to theparticular performance properties of a thermoset material, thermoplasticmaterials are preferably used for coating. On the one hand, they combinewell with the surface and, on the other hand, they are readily formableor smoothable.

[0021] In order to enable commercially available hot melt applicators tobe used to apply the thermoplastic coating material, the thermoplasticmaterial should be sufficiently free-flowing during application. Goodfree flow of the material is important insofar as it is thus able betterto wet the transfer tape, can be better applied to the narrow surface tobe coated and, in addition, can penetrate better into the pores of thechipboard. To avoid excessive penetration, that side of the coating orthe transport tape remote from the narrow side is cooled, for example bymeans of a cold air blower or a cooling roller. That part of the coatingmaterial in contact with the transport tape thus hardens very quickly sothat it “sinks” less into the surface. This effect can be increased byfoaming.

[0022] In addition, reactive non-thermoplastic materials can be used forcoating. Specific examples of such materials include one-component ortwo-component systems based on polyurethanes, polyesters,poly(meth)acrylates and epoxides. They may contain additives, forexample fillers, pigments, dyes, thixotropicizing agents, catalysts andstabilizers. Decorative edges can be directly obtained by addingpigments or dyes.

[0023] Where reactive coating materials are used, it is important toavoid temperatures at which they harden by crosslinking in the mixingand metering units or during application and smoothing.

[0024] In addition, the coating material should lend itseld to bonding,i.e. it should have a high affinity for the decorative layer applied atthe same time or at a later stage.

[0025] The coating material or the wood-based material contains a“blowing” agent which forms pore-forming gases under the effect of theheated metal shoe. In the most simple case, the blowing agent is areadily volatile substance, for example water, a low-viscosityhydrocarbon or halogenated hydrocarbon. However, it may also be asubstance which decomposes and forms gases, for exampleN,N′-azobisisobutyronitrile, toluene-4-sulfonyl semicarbazide,4,4′-hydroxy-bis-(benzenesulfone hydrazide) or azodicarboxylic aciddiamide.

[0026] However, these pore-forming gases may also be formed during thehardening reaction of the reactive coating composition, for example CO₂during the reaction of isocyanates with water. The hardening temperatureand the hardening time are entirely dependent on the processingtechniques and the coating materials used. In general, the temperaturesare in the range from 50 to 250° C. and, more particularly, in the rangefrom 80 to 180° C. while the contact times are between 1 and 300 secondsand, more particularly, between 2 and 20 seconds. Accordingly, thisaffords the advantage that the wood-based material can be continuouslyprocessed or at best only brief stoppages are necessary, depending onthe layer thickness of the coating material.

[0027] The necessary heating unit may consist of a large metal block(heating shoe) which is heated to the necessary temperature by, forexample, a heating cartridge. It is of advantage to use several smallheating shoes, which can be variably adapted to varying contoured edgesurfaces, rather than a single large heating shoe. The metal block canbe shaped in such a way that it also has a smoothing effect (heatingshoe). If the smoothing composition to be hardened is applied by thetransfer tape to a contoured chipboard edge, the heating shoe has to beshaped in such a way or the group of small heating shoes has to bearranged in such a way that the flexible transfer tape is pressed ontothe edge following all the contours or with any other required shape. Itis important to use a flexible transfer tape which withstands theconsiderable heating involved in long-term use and transfers the heat tothe smoothing composition to be hardened. It has been found in practicethat glass-fibre- or Kevlar-reinforced teflonized bands (usable up to260° C.), for example, are suitable. A thin teflonized steel band mayalso be used.

[0028] In addition, it was found in the trials conducted thus far thatthe chipboard with its specific layer structure poses problems insofaras, although the anchorage and hence the adhesion of the smoothingcomposition in the porous middle part are good, adequate adhesion in themore highly compressed marginal zones can only be achieved withlow-viscosity smoothing compositions. Here, it was found that rougheningof the margins leads to a better, satisfactory result. Machineroughening can be carried out inexpensively in a one-pass machine usingcommercially available tools.

[0029] The described coating process may be used in particular forwood-based materials, such as solid wood, chipboards, blockboards,coreboards or plywood. However, the described process may also be usedto coat uneven plastic materials or metals.

[0030] Besides smoothing surfaces, the coating material also offersprotection against mechanical stressing. For example, metal rails on theedges of concrete formwork boards can be replaced by a coating of amaterial which hardens to form a thermoset. Other applications are ofcourse also possible.

[0031] Thus, a decorative layer can be laminated onto the smoothednarrow surface, preferably at the same time as the hardening process.Edges of PVC, ABS, PP or wood (veneer) may be used as the decorativelayer. They may comprise one or more layers (laminate edges).Resin-impregnated paper in particular is used.

[0032] If, for example, a two-component polyurethane system is appliedto the decorative edge material and if this adhesive-coated edge band isbonded to a straight or contoured edge of a chipboard, thethermoset-forming hardening process can surprisingly be accelerated by aheating shoe so that, immediately after leaving the heating unit, it hasadvanced sufficiently for the subsequent steps (milling, capping). Thepolyurethane system is advantageously formulated in such a way that itreacts sufficiently quickly, but does not lead to troublesome hardeningreactions in the mixing head itself. Accordingly, subsequent heatingwith the heating shoe enables an edge band to be bonded in-line onto theedge of the chipboard. Since the PUR adhesive hardens to form athermoset, the bond is highly resistant to heat and water, as requiredfor example for kitchen and bathroom furniture. The expert would nothave expected the combination of a quick-curing, but readily processablereactive coating material with a heated heating shoe to allow such rapidmachine bonding that the subsequent processing steps, such as millingand capping of the edge overhang, can be carried out shortly thereafter,thus ensuring continuous production. In addition, the bond establishedis remarkably impervious.

[0033] The process according to the invention is applicable to thesurface improvement of a wood-based workpiece not only by the edgebanding of straight edges and by softforming (curved edges), but also bypostforming, profile jacketing and 3D jacketing.

[0034] The following observations are intended to explain these threeprocesses. If, in the case of an elongate wood-based board, one of thetwo major surfaces and at least one of the two longitudinal edgesrounded in some way are laminated with one and the same coatingmaterial, preferably a decorative laminate, such as CPL or HPL, theprocess in question is known as postforming. If not only the twolongitudinal edges but also the surface of a long narrow wood-basedmaterial are profiled and if this profile workpiece is laminated with adecorative coating material, the process in question is known as profilejacketing. In so-called 3D jacketing (3D =three-dimensional), alsocommonly known as thermoforming, veneer or a thermoplastic film, mostlyof PVC, is laminated in thermoforming presses—with or without amembrane—to a wood-based panel which is decoratively profiled on allfour sides and possibly even on its flat surface.

[0035] Workpieces of a wood-based material with an uneven surface can beused in the above-mentioned processes after smoothing (particularly ofthe rough porous sides by the described process).

[0036] In all the processes mentioned, however, smoothing and laminationwith a decorative coating material can be carried out simultaneously.This is shown in Example 4 for edge banding.

[0037] The invention is illustrated by the following Examples.

[0038] I Starting materials

[0039] 1. Reactive adhesive 1 is a one-component polyurethane based onMDI and polypropylene glycol.

[0040] 2. Reactive adhesive 2 is a two-component CaCO₃-filledpolyurethane based on crude MDI and castor oil.

[0041] 3. Reactive adhesive 3 is a two-component system of a bisphenol Adiepoxide and an amine hardener with an accelertator.

[0042] 4. Reactive adhesive 4 is a two-component polyester adhesive ofan unsaturated polyester and benzoyl peroxide, dimethyl-p-toluidine andcobalt octoate.

[0043] 5. Reactive adhesive 5 is a two-component methacrylate adhesiveof polyurethane dimethacrylate and benzoyl peroxide.

[0044] The following tests were carried out with the above smoothingcompositions.

EXAMPLE 1

[0045] A strand of the smoothing composition (see Table 1 below) isapplied centrally to the rounded edge of a 19 mm chipboard with asoftforming profile (round profile with a radius of 20 mm). The quantityis gauged in such a way that the edge is completely covered after thesubsequent pressing step.

[0046] The edge coated with the smoothing composition is pressed againsta heated metal rail of which the surfaces are adapted to the roundedchipboard edge. To prevent adhesion to the metal rail, silicone paper isused as an interlayer. The surface temperature of the heated metal rail:see Table 1.

[0047] After the contact time shown in Table 1, the chipboard workpieceis unclamped and pressed briefly against a similarly rounded cold metalrail to accelerate cooling.

[0048] With all the adhesive systems mentioned, the smoothingcomposition is cured. The edge bead formed can be completely removedwith a razor blade. TABLE 1 Smoothing Temperature Contact composition ofmetal rail time Result Reactive adhesive 1 150° C. 10 secs. Hard,completely smooth surface Reactive adhesive 2 150° C. 4-5 Hard,completely secs. smooth surface Reactive adhesive 3 150° C. 15 secs.Hard, completely smooth surface Reactive adhesive 4 Hard, completelysmooth surface Reactive adhesive 5 Hard, completely smooth surface

EXAMPLE 2

[0049] The two-component PUR adhesive is applied as smoothingcomposition to the surface of a flexible material—as listed in Table2—which has an adhesive-repellent coating. The quantity is gauged insuch a way that the edge is completely covered after the subsequentpressing step.

[0050] The material coated with the smoothing composition is pressedagainst the surface of the rounded chipboard edge (as described inExample 1) and cured by a heated metal rail (as in Example 1). Thesurface temperature of the heated metal rail: see Table 2.

[0051] After the contact time shown in Table 2, the chipboard workpieceis unclamped and pressed briefly against a similarly rounded cold metalrail to accelerate cooling.

[0052] The flexible material can be completely removed from the hardenedchipboard edge. TABLE 2 Substrate Smoothing Temperature Contact materialcomposition of metal rail time Result Silicone Reactive 150° C. 10 Hard,completely paper adhesive 1 secs. smooth surface Silicone Reactive 150°C. 4-5 Hard, completely paper adhesive 2 secs. smooth surface TeflonizedReactive 150° C.  7 secs. Hard, completely Kevlar cloth adhesive 2smooth surface Teflonized Reactive 150° C.  3 secs. Hard, completelysteel band adhesive 2 smooth surface (0.05 mm)

EXAMPLE 3

[0053] A strand of the colored smoothing composition (see Table 3 below)is applied centrally to the rounded edge of a 19 mm chipboard with asoftforming profile (round profile with a radius of 20 mm). The quantityis gauged in such a way that the edge is completely covered after thesubsequent pressing step.

[0054] The edge coated with the colored smoothing composition is pressedagainst a heated metal rail of which the surface is adapted to therounded chipboard edge. To prevent adhesion to the metal rail, siliconepaper is used as an interlayer. The surface temperature of the heatedmetal rail: see Table 3.

[0055] After the contact time shown in Table 3, the chipboard workpieceis unclamped and pressed briefly against a similarly rounded cold metalrail to accelerate cooling.

[0056] With all the adhesive systems mentioned, the smoothingcomposition is cured and has a uniformly colored surface. The edge beadformed can be completely removed with a razor blade. TABLE 3 SmoothingTemperature Contact composition of metal rail time Result Reactiveadhesive 1 150° C. 10 secs. Hard, completely smooth white surfaceReactive adhesive 2 150° C. 15 secs. Hard, completely smooth bluesurface 2C epoxy adhesive 3 150° C.  4 secs. Hard, completely with 5%titanium smooth white surface dioxide

EXAMPLE 4 (edge banding)

[0057] The adhesive is applied to the surface of 0.6 mm thick oak veneeror a film as listed in Table 4 (paper- or plastic-based).

[0058] The material coated with the adhesive is pressed against thesurface of the rounded chipboard edge (as described in Example 1) andcured by a heated metal rail (as in Example 1). The surface temperatureof the heated metal rail: see Table 4.

[0059] After the contact time shown in Table 4, the chipboard workpieceis unclamped and pressed briefly against a similarly rounded cold metalrail to accelerate cooling.

[0060] The flexible material is completely bonded to the roundedchipboard edge. The adhesive joint in the edge zone is not visible withthe naked eye. The surface of the material stuck on is extremely smoothand has no undulation. TABLE 4 Smoothing Temperature Contact Coatingmaterial composition of metal rail time Result Oak veneer, 0.6 Reactive150° C. 12 secs. Completely mm adhesive 1 smooth surface DecorativeReactive 150° C.  4 secs. Completely paper film (75 adhesive 2 smoothsurface g/m²) Alkorcell film Reactive 150° C.  4 secs. Completelyadhesive 2 smooth surface Monolayer edge Reactive 150° C.  5 secs.Completely Igrafol (Bausch) adhesive 2 smooth surface

1. A process for improving surfaces, especially narrow surfaces, ofwood-based materials, characterized in that a formable coating materialis applied to the surface, the coating material applied is smoothedand/or modelled and, in contact with at least one heatable metal block,forms closed pores with the smooth surface in tact.
 2. A process asclaimed in claim 1, characterized in that the coating material isreactive and, more particularly, is a one- or two-component system basedon polyurethane, polyester, poly(meth)acrylate or epoxide and hardens incontact with at least one heatable metal block with the smooth surfacein tact.
 3. A process as claimed in claim 1, characterized in that thecoating material remains thermoplastic.
 4. A process as claimed inclaims 1, 2 or 3, characterized in that the metal block has temperaturesof 80 to 180° C.
 5. A process as claimed in at least one of claims 1 to4, characterized in that the contact time with the metal block isbetween 2 and 20 seconds.
 6. A process as claimed in at least one ofclaims 1 to 5, characterized in that a decorative layer is adhesivelyapplied during contact with the metal block.
 7. An assembly for carryingout the process claimed in claims 1 to 6, characterized in that theheating shoe consists of several parts which are adapted to the requiredcontour.